Optical storage media such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are widely employed to store considerable digital data. In order to enhance reproducing quality of the optical storage media, it is required to accurately and quickly read out the stored digital data.
Since some uncontrolled factors adversely affect the optical disks during manufacturing, the signals inevitably decay or are interfered with while being decoded/transmitted. Therefore, it is important to recover the original digital data in the following procedure.
Please refer to FIG. 1. An optical storage medium input apparatus 10 comprises an optical pickup head 11, a reading device 12, a signal line B, an RF amplifier 13, a sampling device 14, a cosine filter 151, an adaptive machine 152, and a decoding device 16. The reading device 12, the signal line B and the RF amplifier 13 are also referred to as a channel response CR. The original digital data Is1 stored in storage medium D, for example a CD or a DVD, are converted into corresponding analog data Is2 through the channel response CR to be inputted to the sampling device 14. In general, the channel response CR includes a full response and a partial response, which are well known in the art and need not be described in details herein.
The analog data Is2 is then transmitted into the sampling device 14. The sampling device 14 principally comprises an analog/digital (A/D) converter, an equalizer and a phase-locked loop circuit (not shown) for converting the analog signals Is2 into corresponding sampled signals Y1˜Yn in digital forms. These sampled signals Y1˜Yn are compensated by the filter 151 and the adaptive machine 152 to generate compensated signals R1˜Rn. As previously described, the sampled signals Y1˜Yn might be distorted. In particular, when the analog data signal Is2 is operated at a high frequency, the sampled signals Y1˜Yn are likely to be deviated from predetermined reference electric levels PR_level due to some non-linear factors. Thus, the cosine filter 151 and the adaptive machine 152 are employed to compensate such deviations, which will be described later, in order to have the output compensated signals R1˜Rn approximate respective reference electric levels. Then, the compensated signals R1˜Rn, which are distributed at respective reference electric levels PR_level, are sent into the decoding device 16. The decoding device 16 comprises at least a Viterbi decoder. The decoding device 16 decodes the compensated signals R1˜Rn and output the decoded data Is3 similar to the original digital data Is1. In such way, the digital data stored in the storage medium D are recovered.
The sampled signals Y1˜Yn are inputted to both of the cosine filter 151 and the adaptive machine 152 for compensation. The process for compensating the above-described deviation comprises steps of determining the respective differences (E1˜En) between the sampled signals (Y1˜Yn) and the compensated signals (R1˜Rn), and feeding the differences E1˜En back to the consine filter 151. The related formulae are as follows:Rn=CYn−4+Yn−2+CYnEn=Yn−RnCn+1=Cn+γEn(Yn−4+Yn)in which, Yn is the nth sampling signal,                Yn−2 is the (n−2)th sampling signal,        Yn−4 is the (n−4)th sampling signal,        Rn is the nth compensated signal,        En is the difference between Yn and Rn,        γ is an operational factor, and        C, Cn and Cn+1 are coefficients.        
Since the coefficients are difficult to be determined by calculation and such calculation sometimes results in no solution, all the sampled signals (Y1˜Yn) might not be correctly compensated to recover the digital data Is1 via the decoding device 16. In other words, the bit error rate (BER) is relatively significant.